Lost lizards validate grim extinction predictions

Predictions that climate change alone could lead to the extinction of more than one-fifth of plant and animal species before the end of the century have often come under fire, and not just from climate-change deniers. Some biologists are sceptical because the predictions are largely based on theoretical models. Now, the most detailed study yet of one group of species – lizards – suggests extinction levels could indeed be as bad as predicted.

Crucially, the new forecast is based on actual data on what is driving lizards to extinction today on four continents.

The international team of researchers led by Barry Sinervo of the University of California, Santa Cruz, found that even though their habitats remained intact, the population of one group of lizards in Mexico has shrunk by 12 per cent since 1975 due to climate change. They found that the reptiles are disappearing because they need to spend more time in the shade to avoid overheating, leaving them less time to find food.

Silent spring

While lizards need to bask in the morning sun to warm up, they have to retreat into the shade later in the day to avoid heat stress. The hotter it gets, the less time they have to forage for food. Surprisingly, though, warmer springs rather than warmer summers are the killer, because this is the time when lizards reproduce and so need lots of extra food.

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The team used its data from Mexico to predict where else lizards might be disappearing. They then compared their predictions with field studies of more than 1000 lizard populations on four different continents. They found that their predictions were accurate. Based on this, they predict that 20 per cent of lizard species will be extinct by 2080.

“This is surprising and very disturbing. None of us expected it,” says Raymond Huey of the University of Washington in Seattle, who wrote an accompanying commentary on the study. “I would have predicted that lizards were less vulnerable to warming.”

Models questioned

Sinervo’s forecast is significant because it is based on real-world studies of populations declining as the climate warms. Until now, most estimates of how many species are threatened by climate change have been based on theoretical studies that look at the climatic and environmental conditions that species need to survive, and overlay this with estimates of how much suitable habitat will remain as the world warms.

“This study is really important because it shows that widespread extinctions associated with climate change are not simply a theoretical construct,” says Chris Thomas at the University of York in the UK.

In a widely cited paper in 2004, Thomas and colleagues estimated that 15 to 37 per cent of terrestrial plant and animal species will be “committed to extinction” by 2050 (Nature, vol 427, p 145). Subsequent studies have reached similar conclusions. There is a lot of uncertainty about such predictions, he says, but it goes both ways&colon; fewer species may die out than predicted, but then again, even more may go extinct.

Fine details

Not all biologists have been convinced, though. One criticism has been that most models are “coarse” in that they do not take into account local details, like mountains and valleys, which might provide climatic refuges for organisms. Then again, studies that do take the finer detail in account do not always predict fewer extinctions, Thomas says.

For instance, coarse models assume it always gets colder higher up but in Europe north-facing slopes and frost hollows can be colder than higher parts of mountains.

Unpublished work by Thomas’s team shows that when this is taken into account for one species of beetle, it disappears even faster than in cruder models.

However, these findings have not been used to make forecasts in the same way Sinervo’s team has done, based on the comprehensive lizard study. “The possibility that climate change will drive a new spate of extinctions across the world is rapidly turning from speculation into fact,” says Thomas.